1. Technical Field
This invention relates generally to articulated pistons for use in heavy duty diesel engine applications, and more particularly to the construction of the skirt.
2. Related Art
Articulated pistons for heavy duty diesel engine applications have the skirt formed separately from the crown and joined in articulated manner through the wrist pin. The crown has an upper ring belt portion in which a plurality of ring grooves are cut for accommodating rings of the piston. A pair of pin boss portions depend from the upper ring belt portion and are formed with an aligned pin bore for receiving a wrist pin to connect the piston to a connecting rod. The crown of the piston is usually fabricated of steel. The articulated skirt of the piston is often fabricated of aluminum. The skirt is formed with a set of pin bores which align with the pin bores of the pin bosses to receive the wrist pin for connecting the skirt to the crown through the wrist pin such that the skirt is able to pivot slightly relative to the crown about the axis of the wrist pin and bores.
The skirt is formed with opposing cylinder wall-engaging thrust surfaces. The thrust surfaces confront the cylinder wall of the bore during the combustion cycle to take up side loads imparted on the piston in order to keep the piston aligned with the bore during operation. The profile of the piston skirt in the vertical direction of reciprocation is typically flat with the ends tapered slightly to provide smooth guidance of the skirt up and down in the piston bore. Under heavy side loading, the skirt flexes laterally, causing distortion of a vertical profile and typically inward cupping of the thrust surfaces toward the middle of the skirt in line with the pin bores. Such introduces high stress in the articulated piston skirt close to their pin bores which is undesirable.
U.S. Pat. No. 4,691,622 discloses a monobloc piston in which the crown and skirt are formed as one piece from the same lightweight aluminum material. In this one piece construction, the skirt extends in axial prolongation of the ring belt and as such the upper end region of the skirt is not free to move and flex in the manner of an articulated piston sleeve along with the rest of the skirt, having the effect of redistributing the stress on the skirt. In other words, the upper end region of a monobloc skirt which is tied to the solid ring belt portion is not able to flex laterally under load in the same manner that the lower free end might.
According to the invention, an articulated piston for an internal combustion engine is provided having a piston crown with a ring belt and a pair of depending pin bosses having axially aligned bores for receiving a pin. The piston includes a separately formed skirt having pin bores which are alignable with the pin bores of the crown for receiving a wrist pin therein to join the skirt to the piston crown in articulated manner. The skirt has a pair of outer thrust surfaces extending longitudinally between upper and lower free ends of the skirt. Each thrust surface has a longitudinal surface profile that is generally symmetrical about a lateral plane containing the axis of the aligned pin bores of the skirt and crown. The longitudinal profiling includes tapered end regions at the upper and lower ends of the skirt and a central depressed region having a lowest valley point substantially in line with the plane the pin bore axis. The profile further provides a pair of longitudinally spaced peaked regions disposed in substantially equal distance from the plane on opposite sides of the recess.
The invention has the advantage of providing a skirt profile having a central recess and a longitudinally spaced peaked regions which distribute loads and stress on the skirt during side loading away from the pin bore regions of the skirt.
The invention has the further advantage of decreasing the overall deformation of the skirt under load in a way that results in a reduction of mechanical stresses. The twin-peaked profile enables the skirt to deform at locations where contact occurs under load for a typical flat profile articulated skirt. By causing the skirt to deform in these regions due to the engineered profile, the contact area is spread out, decreasing the maximum wear load by a factor of about 3. This has the effect of decreasing the occurrence of scuffing and heavy wear marks often imparted to the thrust surfaces of conventional skirts as a result of heavy side loading accompanied by considerable piston skirt deflection.
Another advantage of the twin-peaked, central depression profile of the thrust surfaces is that the depression provides a reservoir for engine oil. Capturing the oil in the depression serves to reduce oil consumption and to channel the lubricant along the depression along the pin bores for providing additional lubricant to the wrist pin joint of the piston.